Rigid or flexible cellular materials, with a polyimide base, have been produced by many methods, either from linear polymers of high molecular weight, or from oligomers, or also from mixtures of monomer reagents. One of the oldest techniques, described, among others, in patents US-A-3,249,561 and US-A-3,883,452 in the name of Du Pont de Nemours, consists in producing a polyimide foam from a polyamide-acid solution in the presence of an agent which decomposes during the heating making possible the formation of the polyimide, by releasing a gas such as carbon dioxide or carbon monoxide. This process is relatively difficult to use because the starting polymer is in diluted solution in a polar organic solvent and it is necessary to control simultaneously the evaporation of the solvent, the imidation reaction and the formation of the cellular structure. With the thermoplastic polyimides such as the polyetherimides, sold by the General Electric company under the trade name of Ultem, the addition of a blowing agent has been used to create the cellular structure, as is described, for example, in patent US-A-4,732,263 in the name of Mobil Oil. Another method, described in patent US-A-4,077,922 of the Upjohn Company, consists in mixing this polyimide type with glass microspheres.
The method which has been the object of a very large number of publications consists in synthesizing the polyimides by making a bis(ortho-acid ester) aromatic react in the place of an aromatic dianhydride on a diamine or on a mixture of several diamines. This reaction provides, by heating at a high temperature, polyimides with release of one water molecule and one alcohol molecule for each imide ring formed. These two volatile compounds are used for blowing agents in the production of cellular materials in polyimides. Since the amount of volatile products released is relatively large, the reaction is generally conducted in two stages. A first reaction of partial polycondensation is performed in a solution to form imide oligomers which are isolated in the form of powder by precipitation in a nonsolvent medium. This fusible powder, optionally mixed with various additives, is placed in a mold and heated above its melting temperature. The expansion of the material is caused by the continuation of the polycondensation reaction.
This production technique has been used with many mixtures of monomers. By way of illustration of this method, patent US-A-3,502,712, in which the reaction of a diester of benzophenonetetracarboxylic-3,3',4,4' acid with metaphenylenediamine is described, can be cited. The use of a diamine mixture, to form more flexible polyimide foams, comprising aromatic diamines and flexible diamines such as the acrylonitrile-butadiene diamines, is, for example, described in patent US-A-4,456,862 or the diaminopoly(dimethylsiloxanes) in patent US-A-4,535,099.
The various methods set forth above make it possible to produce thermostable foams of polyimides having very varied characteristics. They can be rigid, semiflexible or flexible according to the nature of the monomers or the polymers used to prepare them. They generally have an open porosity, but in some cases, the latter can be closed, and they cover a fairly wide range of density and resistance to compression.
But, in a general way, most of these production processes require a strict control of the synthesis conditions of polymers and of the protocol of use of these polymers to obtain reproducible results.
The polyimide resins called "nadimide resins" have as their main application the production of composite materials with a base of glass fabric or of carbon fibers. They have been described in particular in European patent application EP A-283330, in the name of the applicant. Their use is performed with a very precise thermal cycle, first under vacuum, and then under pressure. The polymerization of the nadic double bonds is fairly complex, because several reactions are superposed in the single addition polymerization. One of these reactions is a Diels-Alder retroreaction during which the nadimide ring is transformed into a maleimide ring with release of a cyclopentadiene molecule as diagrammed by equation 3 below: ##STR1##
If the thermal polymerization reaction is performed under pressure, the cyclopentadiene formed in this reaction can be copolymerized with the maleic double bonds and thus enter into the macromolecular structure of the final tridimensional lattice. If, on the contrary, the thermal polymerization reaction is achieved without applying pressure or under vacuum, a part of the cyclopentadiene is eliminated in gas form and the structure of the lattice is slightly different.
It could be interesting to try to use the reaction of formation of cyclopentadiene, which occurs at a high temperature, to assure the expansion of the resin and to form an expanded material from these polymers of nadimide type, the cyclopentadiene thus playing the role of a blowing agent. However, the nadimide resins, in particular those of polyimide type, usually have a molecular weight which is relatively low in number, less than 5,000, and the expanded materials obtained from these products are usually rigid and friable.